5.1.3 Treatment of Other Macros Based on SETF

For each of the ``read-modify-write'' operators in the next figure, and for any additional macros defined by the programmer using define-modify-macro, an exception is made to the normal rule of left-to-right evaluation of arguments. Evaluation of argument forms occurs in left-to-right order, with the exception that for the place argument, the actual read of the ``old value'' from that place happens after all of the argument form evaluations, and just before a ``new value'' is computed and written back into the place.

Specifically, each of these operators can be viewed as involving a form with the following general syntax:

 (operator preceding-form* place following-form*)

The evaluation of each such form proceeds like this:

1. Evaluate each of the preceding-forms, in left-to-right order.

2. Evaluate the subforms of the place, in the order specified by the second value of the setf expansion for that place.

3. Evaluate each of the following-forms, in left-to-right order.

4. Read the old value from place.

5. Compute the new value.

6. Store the new value into place.

decf  pop   pushnew  
incf  push  remf     

Figure 5-9. Read-Modify-Write Macros

• READ-MODIFY-WRITE-EVALUATION-ORDER:DELAYED-ACCESS-STORES